Error when applying readRDS function to Arvados Collection connection in R

Question

I get an error when I try to read a R object from a RDS file which is located on an Arvados platform. First, I use the following code to connect to Arvados and get a file listing:

library('ArvadosR')

# Read ARVADOS_API_TOKEN and ARVADOS_API_HOST
readRenviron("~/.config/arvados/arv.conf")

# Connect to Arvados
arv = Arvados$new()

# Get collection object
coll = Collection$new(arv, "<collection_uuid>")

# List collection content
coll$getFileListing()

When I read a tab delimited file xyz.txt with

df.a = read.delim(coll$get("xyz.txt")$connection("r"))

it works perfectly fine. However, when I try to read an RDS file abc.RDS with

df.b = readRDS(coll$get("abc.RDS")$connection("r"))

I get the following error :

Error in readRDS(coll$get("abc.RDS")$connection("r")) : 
   unknown input format

Reading abc.RDS from a local drive works fine. According to the help page, readRDS accepts "a connection or the name of the file where the R object is saved to or read from".

Answer 1

I could make it work with readRDS(gzcon(coll$get("abc.RDS")$connection("rb"))) .

In help(readRDS) I noticed this paragraph:

"Compression is handled by the connection opened when file is a file name, so is only possible when file is a connection if handled by the connection. So eg url connections will need to be wrapped in a call to gzcon ."